Variable Compliance Wheel

ABSTRACT

A system for adjusting the compliance of a wheel is provided. In one embodiment, wheel segments are adjusted, causing the stiffness of the wheel to change. Such adjustments can be made while the wheel is rotating, allowing the wheel compliance to be changed while a vehicle is in motion.

RELATED APPLICATIONS

This application is a continuation of and claims priority to U.S. patentapplication Ser. No. 16/503,361 filed Jul. 3, 2019 entitled VariableCompliance Wheel, which is a continuation of and claims priority to U.S.patent application Ser. No. 15/817,033 filed Nov. 17, 2017 entitledVariable Compliance Wheel (now abandoned), which is a continuation ofand claims priority to U.S. patent application Ser. No. 14/135,410 filedDec. 19, 2013 entitled Variable Compliance Wheel (now abandoned), whichis a divisional of and claims priority to U.S. patent application Ser.No. 11/574,810 filed Mar. 6, 2007 entitled Variable Compliance Wheel(now U.S. Pat. No. 8,631,844 issued Jan. 21, 2014), which is acontinuation-in-part of and claims priority to International PatentApplication No. PCT/US2005/015478, International Filing Date Jun. 13,2005, entitled Variable Radial And/Or Lateral Compliance Wheel and whichclaims priority to U.S. Provisional Application Ser. No. 60/869,714filed Dec. 12, 2006 entitled Variable Compliance Wheel (now expired),all of which are hereby incorporated by reference in their entireties.

BACKGROUND OF THE INVENTION

The present invention generally relates to a wheel with variablecompliance having many applications, including ground vehicle wheels,printing press rollers, material processing and handling equipment. Moreparticularly, the present invention relates to a wheel that includes arim having a center axis of rotation and a plurality of wheel segmentsengaged with the rim and connecting to a radial band appropriate for theintended usage of the wheel, where the rim and plurality of wheelsegments are adapted to rotate about the center axis, and where theattachment points to the rim of the plurality of wheel segments can bemoved in the direction of the axis of rotation. In the example of aground vehicle application, the radial band includes tread elements toimprove vehicle traction in wet or rough road surfaces or in complexterrain. For material handling applications, the radial band may have asmooth surface to apply even pressure to a printing medium, for example,or may contain striations or tread elements as required.

Various mechanisms currently exist for varying the ground contactpressure of a tire while a vehicle is being driven. This capabilityallows a vehicle to traverse soft soils by lowering the pressure withinthe tire, without compromising on-road performance. This is commonlyachieved by varying the pressure within the carcass of a tire throughthe control of an air valve mounted on the wheel that can vent thetire's internal air to atmospheric pressure or to the pressure producedby an onboard air compressor plumbed through an airtight rotary seal.The commercial term for such a system is Central Tire Inflation System(CTIS). One such system is U.S. Pat. No. 5,553,647, “Central tireinflation system,” (Miroslav), the contents of which are herebyincorporated by reference, that describes a pressure air source,plumbing, valve, pressure sensor, and control system for varying theinternal air pressure of a tire while driving. The shortcomings of thisdesign are complexity, sensitivity to the elements, cost, and inabilityof the system to maintain adequate tire pressure when the tire is badlydamaged.

Several patents have addressed this latter concern of pneumatic tirevulnerability with runflat inserts (U.S. Pat. No. 6,263,935, “Radial plypneumatic runflat tire,” (Oare, et. al.)) or tires that do not rely onfluidic pressure for load carrying, a.k.a. non-pneumatic tires, (U.S.Pat. No. 6,431,235, “Non-pneumatic tire and rim combination,” (Steinke,et. al.)), the contents of both of these patents hereby incorporated byreference. The shortcomings of this system are weight and fixed tirestiffness.

The present invention describes a novel way of combining the benefits ofvariable tire stiffness with a damage tolerant tire design.

SUMMARY OF THE INVENTION

The present invention provides a variable stiffness wheel that can beused in a variety of applications, such as for support of ground vehicletraverse over a variety of terrain or for conveying materials (e.g.,airport baggage handling). In one preferred embodiment, the variablestiffness wheel includes a plurality of wheel segments whose attachmentsto the center rotating rim (inner and outer) can be moved in thedirection of the axis of rotation, thereby changing the tension of theseelements, which is convenient, in one example, for tuning the groundcontact pressure of the tire for the terrain being traversed. The innerattachment being accommodated via a sliding flange located towards thevehicle and the outer being accommodated via another sliding flangelocated away from the vehicle. Additionally, this method can be used tovary other wheel stiffness parameters such as vertical stiffness,lateral stiffness, torsional stiffness (about the axis of rotation orabout an axis perpendicular to the axis of rotation) each of which canaffect overall behavior of the system the wheel is used in (e.g.,vehicle performance).

With the inner and outer wheel segment attachment points close togethertensioning the spokes, the vertical stiffness of the wheel is increased.In the example of the vehicle wheel, this increased stiffness generallyprovides excellent on-road performance (cornering, steering feel, lowrolling resistance, etc.) and increased payload carrying capacity andincreased durability at high speeds.

With the inner and outer wheel segment attachment points spread apartfrom one another, the vertical stiffness of the wheel is reduced,enlarging the contact patch of the wheel with, for example, the ground,baggage, or other material. In the example of a printer, the stiffnessof the print roller can be modulated to compensate for plate wear,extending the life of the plate and improving the throughput of thepress. In the example of the vehicle wheel, the tire/terrain envelopingperformance is improved, while the lower ground pressure gives thevehicle better off-road mobility on soft soils like mud, sand, and snow.

Continuing with the example of the vehicle, the inner and outer wheelsegment attachment points spread far apart from one another which allowsthe vehicle to be lowered, thereby facilitating transportation in lowclearance vehicles like aircraft. The lower ground pressure of thisconfiguration is also beneficial to ramps and cargo floors that havestrict limits on floor loading pressure due to floor structurelimitations imposed by weight constraints, as is the case with manyaircraft.

The inner and outer wheel segment attachment points can be varied fromthe maximum and minimum spacing while driving to suit the immediateneeds of the vehicle.

Multiple inner and outer wheel segments can be stacked to produce awheel with varying radial stiffness across the width of the wheel. Thisis beneficial for improving the lateral performance of the wheel bycontrolling the wheel's dynamic camber. In the example of wheelssupporting a conveyer belt, this varying stiffness across the wheelwidth may help direct or steer baggage in a desired direction. In theexample of a printer, the varying radial stiffness of a printer rollermay direct paper in a desired direction or area, such as continuouslyadjusting the registration of multicolor press runs. This reduces thewaste and rework associated with misregistered press output.

In the example of the vehicle, the dynamic camber can adjusted duringcornering to improve maneuvering performance. This can also bebeneficial for changing the heading of the vehicle while driving withlittle or no steering of the tire. By making the stiffness of theinnermost pair of inner and outer wheel segments stiffer than theoutermost pair, resulting forces at the wheel/road contact patch willserve to pull the vehicle in the direction of the outermost pair ofwheel segments. Further, by reversing the relative stiffness between thetwo sections, the wheel can force the vehicle in the opposite direction.This has the potential of simplifying the steering system, reducing costand weight, improving the durability of the suspension/steering systemby eliminating vulnerable steering links, and reclaiming the sweptvolume lost to the tire as it steers for other vehicle components orcargo. Additionally, the radial wheel stiffness, when modulated at ahigh rate, can be used to counteract vehicle pitch and heave vibrations,augmenting or even replacing the vibration isolation functions of thevehicle's suspension system.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is an isometric view of a preferred embodiment of a wheel ofvariable stiffness of the present invention;

FIG. 1B is a side view of the variable stiffness wheel of FIG. 1;

FIG. 2 is a detail isometric view of a cutaway of the variable stiffnesswheel of FIG. 1, showing the movable elements of the wheel with aportion of the wheel removed;

FIG. 3 is a cross sectional view of the variable stiffness wheel of FIG.1;

FIG. 4 is a side view of a second preferred embodiment of a wheel ofvariable stiffness of the present invention;

FIG. 5A is an isometric cutaway view of the variable stiffness wheelshown in FIG. 4;

FIG. 5B is an enlarged detail view of FIG. 5A;

FIG. 6 is a cross sectional view of the preferred embodiment of FIGS. 5A and B;

FIG. 7A illustrates a speed-sensitive automatic stiffness adjustmentmechanism according to a preferred embodiment of the present invention;

FIG. 7B illustrates a speed-sensitive automatic stiffness adjustmentmechanism according to a preferred embodiment of the present invention;

FIG. 8A describes a pneumatic stiffness adjustment mechanism which canbe employed with either preferred embodiment of a wheel of variablestiffness of the present invention;

FIG. 8B describes a pneumatic stiffness adjustment mechanism which canbe employed with either preferred embodiment of a wheel of variablestiffness of the present invention;

FIG. 9 illustrates a top view of a luggage carrier according to apreferred embodiment of the present invention;

FIG. 10A is a front view of a variable stiffness wheel with amanually-operated stiffness adjustment mechanism according to apreferred embodiment of the present invention;

FIG. 10B is a partial cross sectional view along line B of FIG. 10A;

FIG. 10C is a partial cross sectional view along line C of FIG. 10A;

FIG. 11 is a partial cross sectional view along line A of FIG. 10A;

FIG. 12A is an exploded isometric view of the wheel center andmanually-operated stiffness adjustment mechanism of FIG. 11; and

FIG. 12B is an additional exploded isometric view of the wheel centerand manually-operated stiffness adjustment mechanism of FIG. 11.

DETAILED DESCRIPTION

FIGS. 1A, 1B, 2, and 3 illustrate a first preferred embodiment of avariable stiffness wheel 10. The variable stiffness wheel 10 includes arim 11, fixed flanges 12 b and 13 b and movable flanges 12 a and 13 a(also referred to as rings, plates and collars in this specification),and a plurality of wheel segments 17, 18 (e.g., two oppositely angledspokes 20) engaged with the flanges 12 a, 12 b, 13 a, 13 b and an outertraction element 16.

The rim 11 includes an inner flange (not shown) which locates an adaptorplate and thereby allows the wheel 10 to be mounted on a spindle (notshown) which allows the variable stiffness wheel to rotate about thecenter axis. Preferably the spindle and mounting mechanism is similar towheel mounting mechanism of present vehicles (e.g., secured by lugnuts). While the variable stiffness wheel 10, as well as otherembodiments of the present invention, are described as being used on avehicle, it should be understood that these embodiments can be used onalmost any device that may utilize a wheel, such as to support aconveyer belt or as a roller for a printer.

As best seen in FIGS. 2 and 3, the wheel segments 17, 18 are distributedevenly around the rim 11. In the illustrated preferred embodiment, thewheel 10 includes about one hundred and twenty wheel segments 17 and 18containing four different wheel segment planes (i.e., the planes formedby each spoke 20 of the segment 17 and 18). It should be obvious toanyone skilled in the art that the variable stiffness wheel 10 mayinclude fewer or more wheel segments 17, 18 and fewer or more wheelsegment planes.

The variable stiffness wheel 10 also includes sliding guides 15 engagedwith the rim 11 and flanges 12 a, 12 b, 13 a, 13 b (the flanges 12 a and13 a defining the movable wheel segments and the flanges 12 b and 13 bdefining the nonmoving wheel segments) which affix the base of theplurality of wheel segments 17, 18 (i.e., the base of the spokes 20) tothe rim 11. Flanges 13 b and 12 b are positively affixed to the wheel torestrict linear movement along the axis of rotation. Flanges 12 a, 13 aslide along the guides 15 and are loaded (i.e., their position ischanged) with preload adjusters 14 that are in contact with the flanges12 a, 13 a and the rim 11. In this respect, as the preload adjusters 14(which move via actuators, not shown in these Figures) pull on theflange 13 a, force is transferred through guides 15 to flange 12 a,causing both flanges 12 a and 13 a to move towards the preload adjuster14. Similarly, the preload adjusters 14 release their load, causing theflanges 12 a and 13 a to move away from the preload adjusters 14.

In an alternate preferred embodiment, all rings may be fixed in place,may slide relative to each other or may include some combinationthereof. For example, the inner flanges 13 a and 12 b may be fixed inplace while the outer flanges 12 a and 13 b may slide relative to therim 11. Further, each of the wheel segments 17, 18 can be configured tonest between adjacent wheel segments 17, 18 or be stacked in line witheach other.

FIG. 2 illustrates a detail isometric view of the variable stiffnesswheel 10 with the traction element 16 sectioned away to illustrate therim 11, sliding rings 12 a, 12 b, 13 a, 13 b, and the plurality of wheelsegments 17, 18. FIGS. 2 and 3 illustrate the variable stiffness wheel10 in one particular preload position, where the spacing between therings 12 a, 12 b, 13 a, 13 b is at a nominal position. Where the ringpairs (12 a, 12 b and 13 a, 13 b) are closest together, the radialpreload on the wheel segments 17, 18 will be at its greatest. Thisresults in the highest radial stiffness of the wheel. Where the ringpairs (12 a, 12 b and 13 a, 13 b) are furthest apart, the radial preloadon the wheel segments 17, 18 will be at its lowest and the angle of thewheel segments 17, 18 is at their greatest angle with respect to thevertical applied load. Conversely, the lowest preload and weakest wheelsegment angle provides the lowest radial stiffness of the wheel.

In the present preferred embodiment, each wheel segment 17, 18 includestwo oppositely angled spokes 20 which connect to one of the flanges 12a, 12 b, 13 a, 13 b and to the traction element 16. Preferably thespokes 20 are coupled to the flanges 12 a, 12 b, 13 a, 13 b and to thetraction element 16 with either adhesive or mechanical fasteners, or byovermolding traction element 16 onto spokes 20. The spokes 20 cancomprise a variety of rigid or semi rigid materials such as polymer orcomposite material.

The preload adjusters 14 preferably include an actuator (not shown inthese Figures) which may be controlled by the vehicle or manuallyadjusted by a user at the wheel itself. In one example, the linearactuator may be a pneumatic actuator driven by a CTIS. In anotherexample, the linear actuator may be an electrical actuator that receivespower through a slip ring connection (e.g., similar to the slip ringconnections used for communication and power in the turret of a tank) toa chassis electrical system. In yet another example, a linear actuatormay be a screw positioned through the rim 11 and connected to theflanges 12 a, 12 b, 13 a, 13 b, thereby causing the flanges to moveaxially.

Preferably, the vehicle includes a control system (e.g., amicroprocessor and control software) for monitoring vehiclecharacteristics such as speed, wheel slippage (e.g., loss of traction onan icy terrain), roughness of terrain, etc., and adjusts the wheelfirmness according to preset firmness profiles during vehicle operation.Preferably, a slip ring connection, as known in the art, can be used forcommunicating or controlling the mechanisms of the wheel 10. In a morespecific example, as the vehicle monitors the increasing speed, themicroprocessor executing the control software of the vehicle thenincreases the firmness of the wheel to provide more desired vehiclehandling at the higher speed. In an even more specific example, thecontrol software of the control system may include multiple speed rangesso that when the vehicle is traveling at a speed within a predeterminedspeed range (e.g., between 1 and 20 MPH) the control system sets apredetermined tire firmness.

The control system may also be used for steering the vehicle by onlymodifying the firmness of a portion of the wheel (e.g., changing thefirmness of half of the wheel). Similarly, the control system can adjusta portion of each wheel's firmness to improve handling characteristicsof the vehicle, such as handling when cornering.

In other alternate preferred embodiments, the preload adjusters 14 maybe actuated through other linear or rotary electromechanical, fluidic,magnetic, or other mechanisms of exerting a force at the base of themovable rings 13 a, 12 a. In another alternate preferred embodiment, thewheel segments 17, 18 may be directly actuated radially orsemi-radially, similar to a camera shutter.

In another alternate preferred embodiment, each spoke 20 includes aninner lumen filled with pressurized media 20 a. The pressure of themedia within the lumen is increased or decreased to respectivelyincrease or decrease the stiffness of the spoke 20, and therefore adjustthe softness of the wheel 10.

In another alternate preferred embodiment, each spoke may be composed ofshape memory alloys to increase or decrease the firmness of the spoke20. For example, the shape memory alloy may include two predeterminedshapes such as a straight and curved shape or two different radii ofcurve shapes. Applying power to the shape memory alloy distorts thespokes 20 between the two predetermined shapes or alternately to curvesin between the two predetermined shapes. In this respect, the firmnessof the wheel can be adjusted.

In another alternate preferred embodiment, artificial muscles or similarcontracting materials (e.g., biomaterials) may be used as a linearactuator as part of the preload adjuster to move the flanges 12 a, 12 b,13 a, 13 b between different positions.

Other mechanisms of adjusting preload tension/compression on theplurality of wheel segments may also include utilization of smartmaterials like artificial muscles biomaterials, or the replacement ofwheel segments with linear or rotary actuators (e.g., as discussed inthe preferred embodiment of FIGS. 7A and 7B).

FIGS. 4, 5A, 5B, and 6 illustrate another preferred embodiment of thevariable stiffness wheel 110, comprising one or more toroid-shaped spokerings 114 composed of a plurality of curved spokes 124. One end of eachspoke 124 of the spoke ring 114 is connected to a spoke collar 116 whichslidably engages the wheel rim 118. The other end of each spoke 124 isconnected to the traction element 112, preferably by adhesive,mechanical fasteners, or by overmolding (e.g., overmolding tractionelement 112 onto the spokes 124).

As seen best in FIGS. 5A, 5B and 6, the wheel 110 includes spoke collars(similar to the flanges or rings described in previous embodiment ofthis specification) which slide in the direction of the axis of rotationof the rim 118. As the spoke collars 116 slide, the end of the spokes124 connected to the collar 116 also slides, thereby changing the curveof the spokes and modifying the firmness of the wheel 110.

The wheel 110 is preferably mounted to a vehicle by a mechanismpresently known in the art. For example, a wheel center 120 is affixedto a vehicle's wheel spindle (not shown), transmitting torque from thewheel spindle through wheel center 120 to the spoke collar 116 throughthe sliding guides 122. Torque then is transmitted through the spokering 114 to traction element 112 which is in contact with the roadsurface, imparting braking and tractive forces to the vehicle. Wheelstiffness is increased by exerting a lateral force to the toroid-shapedspoke rings 114 in the direction of the axis of rotation of the wheel110, away from the plane of symmetry (shown in FIG. 4). Wheel stiffnessis reduced by exerting a lateral force to spoke rings 114 towards theplane of symmetry.

FIG. 7A and FIG. 7B illustrate another preferred embodiment according tothe present invention of a wheel for automatically increasing radialspoke stiffness according to increasing vehicle speed. Generally, thisstiffness adjustment is achieved by harnessing the force of a mass 210(or optionally a plurality of masses) which rotates with the wheel,thereby exerting force on the wheel as it rotates to change theconfiguration of the spokes.

At least one tension mass 210 is attached to outer spoke collar 212 byflexible cable 214. The flexible cable 214 is routed over a reactionpulley 216 which is attached to wheel rim 218. When the wheel rotatesslowly (as seen in FIG. 7A), the force exerted on outer spoke collar 212from the mass 210 is low, allowing outer spoke collar 212 (i.e., movablecollar) to be pulled closer to inner spoke collar 220 (non movablecollar) by the keeper spring 226. This results in relatively low tensionin each spoke ring 222 which maintains the wheel in a “soft”configuration with an enlarged ground contact patch size of tractionelement 224. As the rotational speed of the wheel increases (as seen inFIG. 7B), the tension mass 210 exerts an increasingly larger force onflexible cable 214, forcing the outer spoke collar 212 to move away frominner spoke collar 220. The movement of outer spoke collar 212 increasesthe tension in each spoke ring 222, increasing the wheel firmness anddecreasing the ground contact patch size of traction element 224.

FIG. 8A and FIG. 8B illustrate another preferred embodiment according tothe present invention for pneumatically adjusting the stiffness ofspokes 320. An air spring 310 (and optionally a plurality of airsprings) is affixed to wheel rim 312 and to the outer spoke collar 314.All air springs 310 are in fluid communication with a centralpressurized air source (not shown) via pneumatic tubing 316. When air issupplied to pneumatic tubing 316, the air springs 310 inflate and forcethe outer spoke collar 314 to move closer to the inner spoke collar 318(i.e., moving from the position illustrated in FIG. 8A to the positionillustrated in FIG. 8B). As with previously described embodiments, thismovement of the outer spoke collar 314 decreases the tension in spokering 320 and increases the ground contact patch size of traction element322 (i.e., decreases firmness of the wheel).

When a pressure relief valve (not shown) is opened, air flows out of airsprings 310 and into pneumatic tubing 316 and is exhausted to atmospherethrough pressure relief valve (not shown). The deflated air springs 310allow the outer spoke collar 314 to move away from inner spoke collar318 (to the position seen in FIG. 8A), increasing the tension in spokering 320 and decreasing the ground contact patch size of tractionelement 322. This relationship may appear counterintuitive when comparedwith a pneumatic time, however this preferred embodiment of the wheelremains stiff if left in its native shape and will become more compliantby pushing the spoke rings inward to decrease the spoke tension.

Preferably, the pressurized air for the air springs 310 is providedthrough a hollow, pressurized vehicle axle spindle which couples andthereby seals to the wheel similar to currently known central tireinflation systems. This sealed region of the wheel is in communicationwith pneumatic tubing 316, allowing the vehicle (e.g., a computer andsoftware within the vehicle) to pneumatically control the air springs310 and thus the firmness of the wheel.

It should be understood that many of the elements described in theembodiments of this specification can be mixed or incorporated withother embodiments set forth in this specification without departing fromthe present invention.

Referring to FIG. 9, a conveyor device 400 consists of a plurality ofrollers 402 with a variable lateral compliance capability, as describedin various embodiments of the present specification. However, since therollers 402 typically have a greater relative width compared withwheels, a plurality of spoke segments may be included along the lengthof rollers 402. The rollers 402 may be configured to freely spin or maybe motorized for propelling luggage 404 in the direction shown with thearrow.

When a sensor (not shown) detects a piece of luggage 404 which should berouted to the leftmost conveyor chute 406, or alternately a user wishesto change the route of the luggage, the lateral compliance of rollers402 is modulated. In this respect the course or direction of luggage 404is changed. Similarly, the lateral compliance of rollers 402 may bemodulated to direct luggage to the rightmost conveyor chute 408.

FIGS. 10A, 10B, 10C, 11, 12A and 12B illustrate another preferredembodiment according to the present invention for manually adjusting thestiffness of spokes 510. Generally, a handle 530 rotates a central cablespool 522 which increases or decreases a cable tension on outboard innerspoke collar 524 and inboard inner spoke collar 520, thereby adjustingthe tension of the spokes 510.

The inboard inner spoke collar 520 is connected to the cable spool 522by a flexible cable 512 (or optionally a plurality of flexible cables).The flexible cable 512 is routed over reaction pulleys 514 between thewheel rim 516 and wheel center 518. Similarly, the outboard inner spokecollar 524 is connected to cable spool 522 by a flexible cable 512 (oroptionally a plurality of flexible cables). The flexible cable 512 isrouted over additional reaction pulleys 514 between wheel rim 516 andwheel center 518.

In this respect, as the spool 522 winds the cable 512, the pulleys 514support the increased cable tension that moves both inner spoke collars520 and 524 toward the center of the wheel. Thus, the tension of thespokes 510 is modified, similar to the previously described embodimentsof this specification.

Referring to FIGS. 11-12B, the cable spool 522 is attached to the wheelcenter 518 by fastener 526 and prevented from rotating with respect towheel center 518 by engagement of a shear pin 528 (or optionally aplurality of shear pins) with wheel center 518. The shear pin 528 isattached to handle 530 and is prevented from disengaging from wheelcenter 518 by helical spring 532.

When the user wishes to increase spoke tension, the operator pulls thehandle 530 laterally outward, disengaging the shear pin 528 from thewheel center 518. After shear pin 528 is disengaged, the operatorrotates the handle 530, causing the cable spool 522 to rotate. When thecable spool 522 rotates, the flexible cable 512 is pulled onto the cablespool 522, drawing the outboard inner spoke collar 524 and the inboardinner spoke collar 520 towards each other and increasing the tension inthe spokes 510. When a desired spoke tension is reached, the operatorpushes the handle 530 laterally inwards, re-engaging shear pin 528 withthe wheel center 518.

Although the invention has been described in terms of particularembodiments and applications, one of ordinary skill in the art, in lightof this teaching, can generate additional embodiments and modificationswithout departing from the spirit of or exceeding the scope of theclaimed invention. Accordingly, it is to be understood that the drawingsand descriptions herein are proffered by way of example to facilitatecomprehension of the invention and should not be construed to limit thescope thereof.

What is claimed is:
 1. A rotatable wheel for traversing a surfacecomprising: a rim; a plurality of radial elements extending from therim; the plurality of radial stiffening elements composed of a shapememory alloy; the plurality of radial stiffening elements being movablebased on conditions of the surface being traversed.
 2. The rotatablewheel of claim 1, wherein a flexible tread band circumscribes theplurality of radial stiffening elements.
 3. The rotatable wheel of claim1, wherein the radial stiffening elements are curved.
 4. The rotatablewheel of claim 1, wherein the radial stiffening elements form atoroid-shape ring around the rim.
 5. The rotatable wheel of claim 1,wherein ends of the radial stiffening elements are disposed at differentpositions along a thickness of the rim.
 6. The rotatable wheel of claim1, wherein the radial stiffening elements are adjustably positionedalong an axis of the rim to thereby adjust a firmness of the rotatablewheel.